Science, Maths & Technology

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# 3.2 Ice sheets

## Activity 3 Global sea level rise

Timing: Allow approximately 10 minutes

Which of the following can contribute to global sea level rise when it melts?

a.

land ice (ice sheets and glaciers resting on land)

b.

sea ice (ice floating in the oceans)

c.

neither land ice nor sea ice

d.

both land ice and sea ice

### Discussion

Only the melting of land ice changes global sea levels: melting sea ice does not change sea level.

When ice on the land melts and flows into the seas, this raises sea levels because it adds a new volume of water to the ocean (Figure 8).

Ice is less dense than water, which is why it floats. When floating ice melts, it forms a smaller volume of water than the original volume of ice. In fact, the volume of water formed is exactly the same as the volume of ice that was below the water surface when it was floating, so no change in sea level occurs.

Figure 8 Floating sea ice does not increase sea level when it melts, because the volume of sea ice which is underwater is the same as the volume of water which is left when the floating sea ice melts.

The glaciers and great ice sheets of Greenland and Antarctica (Figure 9) might seem like giant ice cubes, inert but for gradual melting from climate change. In fact, they are dynamic, shifting landscapes, places of delicate balance between the forces that create ice and those that destroy it.

Figure 9 Satellite images of the (a) Greenland and (b) Antarctic ice sheets.

Under its own immense weight, ice flows continuously downwards, towards the sea if possible. Ice is lost whenever icebergs break off into the sea – along ice sheet coasts – or whenever the surface is warm enough to melt it. Ice is constantly replaced as falling snow compacts, or rain and meltwater freeze. It all adds up to an ice ‘mass budget’ that changes with altitude, location, the seasons and long-term climate change.

This dynamic nature of ice means that dramatic images of the end of a glacier collapsing, or a huge iceberg breaking away, are not necessarily caused by climate change. Likewise, these events may not be causing global sea level rise, because glaciers are constantly losing and gaining ice. What matters for climate change and global sea level rise is how fast ice is being lost, and whether the rate of loss is increasing.